142 Biological Chemistry. 



C 6 H 4 (CH 3 ) 2 . (Compare preparation of benzene on p. 136.) 

 This substance on oxidation yields a phthalic acid, in 

 which the carboxyl groups must be attached to carbons 

 separated by another carbon atom that is, it must be 

 the mefa-derivative 



CH/ S |CH 3 CH/\CH 3 CH, 



V " 



CH, 



VCOOH 

 COOH* 



It would follow from the above, that the constitution of 

 any derivative of the type (C 6 H 4 )XY, which could be con- 

 verted into a phthalic acid, could be ascertained. The 

 following may be taken as an example of how such a 

 method can be applied. There are three possible derivatives 

 of benzene in which a hydrogen is replaced by a methyl 

 group and a bromine of the formula C 6 H 4 (CH 3 )Br, which 

 are known as bromotoluenes. If such substances are 

 treated with methyl iodide in the presence of sodium, the 

 following reaction, which is the same as that already 

 described in the case of the aliphatic derivatives, would 

 take place : 



C 6 H 4 (CH 3 )Br + CH,I + Na, = Nal + NaBr + C 6 H 4 (CH 3 ) 2 



A dimethyl benzene would, therefore, be obtained, which 

 on oxidation would yield a phthalic acid. As the three 

 phthalic acids differ from one another in melting points 

 and other properties, it would be quite easy to ascertain 

 the constitution of the bromotoluene from which any one 

 of these acids was obtained. 



Another method for determining the constitution of 

 di-substitution products of benzene is due to Korner. This 

 depends upon the fact that, if a tri-substitution product is 



* Note that it does not matter which (CH 3 ) group is indicated as having 

 undergone oxidation, as the positions are all symmetrical. 



